An apparatus for fitting the tip of a driver bit into the groove on the head of a screw, disengaging the fitted screw from a screw carrier strip by a forward stroke force of the driver bit and driving the screw into a workpiece while rotating the driving bit is publicly known as Japanese Patent Laid-Open No. 7-171770 (JP-A-7-171770). In this screw driving apparatus, in order to solve the problem such a reaction that the driving apparatus is raised when the screw pressed against the workpiece is driven, a moving piston for advancing a driver bit is connected through a spring to the driver bit. The piston can move forward in the way that it compresses the spring even when the driver bit abuts against the workpiece to resist further advance, thereby it preventing the reaction in the direction that the apparatus tends to be raised.
Nevertheless, the above screw driving apparatus described in Japanese Patent Laid-Open No. 7-171770 (JP-A-7-171770) causes additional reaction if a strong spring to meet a strong pressing force is used for high-speed pressing and high-speed driving of screws because a greater pressing force is required for screwing them in after they abut against the workpiece. On the contrary, the use of a weak spring decreases the speed of pressing screws to slow the driving operation. It is also feared that failure in disengaging screws from a screw carrier strip or in fitting of the tip of the driver bit into the groove on the screw head may arise to hinder the driving operation.